1. Field of the Invention
The invention relates to new 3,4-alkylenedioxythiophenes which are substituted with mesogenic groups and their polymeric derivatives (poly-(3,4-alkylenedioxythiophenes)).
2. Brief Description of the Prior Art
It is generally known that π-conjugated polymers display interesting (nonlinear) optical properties because of the considerable delocalization of the π electrons along the main chain. After oxidation or reduction, they are good electric conductors and in the uncharged form they possess good semiconducting properties. They are, therefore, of interest for use in fields such as data storage, optical signal processing, suppression of electromagnetic interference (EMI) and solar energy conversion, and also in rechargeable batteries, light-emitting diodes, field effect transistors, printed circuits, sensors and antistatic materials.
Poly(3,4-alkylenedioxythiophenes) and their derivatives, in particular poly(3,4-ethylenedioxythiophene) and its derivatives are of particular interest, owing to their high conductivities, especially in the cationic (oxidized) form, and their good processability (EP-A 339 340 and L. Groenendaal, F. Jonas, D. Freitag, H. Pielartzik & J. R. Reynolds, Adv. Mater. 12, (2000) pp. 481-494).
Despite these good properties of poly(3,4-ethylenedioxythiophene), there is a need for further improvement to meet the requirements of conductive and semiconducting materials. The conductive or semiconducting materials have to meet very demanding requirements, in particular the ordering requirement, for use in, for example, molecular electronics, solar technology or semiconductor technology.
Hitherto, the prior art has not disclosed means of influencing a high degree of preorientation of the poly(3,4-ethylenedioxythiophene) in a solid phase, since conductive or semiconducting polymers are processed mainly from the liquid phase, e.g. solution, suspension or dispersion.
However, it is envisaged that such preorientation could be achieved by self-organization of the monomers during the polymerization by creating structural prerequisites for short-range order.
In this regard, the prior art discloses a number of attempts to prepare π-conjugated polymers bearing mesogenic groups as substituents, i.e. groups which promote the formation of mesogenic phases. Described hereinbelow are the prior art products and processes and their shortcomings.
Attempts to prepare thiophenes containing mesogenic substituents or their polymeric downstream products have also been described in a few publications. J. Roncali et al. (Adv. Mater. 1994, 6(2), pp. 138-142) describe the synthesis of 4-cyano-4′-(8-(3-thienyl)oxy)biphenyl, its liquid-crystalline behavior and electro-chemical polymerization thereof to form the corresponding polythiophene. Its electrical conductivity is said to be from 0.01 to 0.1 S/cm. In Synthetic Metals 1999, 102, p. 1291, Akagi et al. report liquid-crystalline polythiophenes bearing chiral mesogenic groups as substituents without giving any further information on the electrical properties. Kijima et al. report a cationic, viologen-substituted liquid-crystalline polythiophene and describe its redox behaviour and the electric conductivity of 3.5×10−3 S/cm in the I2-doped state, but this is extremely low with a view to the abovementioned applications (Chem. Letters 2000, pp. 936-937). Yagci et al. describe the preparation of the liquid-crystalline cholesteryl 3-thiopheneacetate which was oxidatively polymerized by chemical and electrochemical means to give the polythiophene having a conductivity of 0.05 S/cm (J. Mater. Sci. 2002, 37, pp. 1767-1775).
All the polythiophenes described in the abovementioned references have two disadvantages caused by their structure. Firstly, the substitution of the thiophene unit exclusively in position 3 leads to secondary reactions in the polymerization, since a reactive H is present as substituent in the 4 position. In particular α,β′-coupling can lead to structural defects, i.e. interruption of the conjugation in the polymer and shortened conjugation lengths, resulting in unsatisfactory conductivities. Secondly, according to the above mentioned publications, the thiophenes which are C-substituted exclusively in the 3 position always exhibit only moderate stabilities of the highly conductive cationic state of the polythiophenes (e.g. after iodine doping). These disadvantageous are well known for analogous polythiophenes which are not liquid-crystalline, cf., for example, Leclerc et al., Macromolecules 1991, 24, pp. 455-459.
In Synthetic Metals 2001, 124, pp. 471-475 Kumar et al. described a 3,4-ethylenedioxythiophene having bulky substituents and its electrochemical polymerization to form the correspondingly substituted poly(3,4-ethylenedioxythiophene), as well as studies on these polymers with a view to improved transparency. Although the bulky substituent can be regarded as a potential mesogenic group, no indications were given of liquid-crystalline behaviour of the monomer or polymer and no studies on this subject were reported.
There is, therefore, a continuing need for 3,4-alkylenedioxythiophenes which bear mesogenic groups as substituents.